I. Field of the Invention
The present invention relates generally to ignition coil assemblies for spark plugs of the type used in internal combustion engines and, more particularly, to an ignition coil assembly of the type used with individual spark plugs.
II. Description of the Prior Art
In the automotive industry, there has been an increased use of distributorless ignition systems in recent years. Such distributorless ignition systems have, in turn, caused an increased use of individual ignition coils for each spark plug of the engine. For example, for a four cylinder engine. four separate ignition coils are utilized. The use of individual ignition coils per spark plug in practice reduces the overall weight of the automobile by eliminating the previously used bulky and heavy single ignition coil used to fire all of the spark plugs in the engine.
The use of individual ignition coils per spark plug enables the ignition coil assembly to be relatively compact and lightweight. Such ignition coils may be mounted around their respective spark plugs in the engine cavity or, alternatively, above the spark plugs and connected to the spark plugs by a suitable electrical connector.
The previously known ignition coil assemblies for individual spark plugs typically comprise a primary coil and a secondary coil in which one coil is nested within the other coil. Typically, the primary coil and secondary coil each comprises a bobbin having the coil wire wound about the bobbin. After the primary and secondary coil wires have been wound about their bobbin, one of the bobbins is coaxially positioned within the other bobbin such that the bobbins are coaxial with each other.
A magnetic core is also conventionally employed to enhance the magnetic coupling between the primary and secondary bobbins. These previously known cores typically comprise a first portion which extends through the primary and secondary coils and a second portion which extends exteriorly around the primary and secondary coils to complete the magnetic circuit. Conventionally, these magnetic cores are constructed from laminated steel in which the laminations are secured together in any conventional fashion.
Each lamination of these previously known magnetic cores is substantially identical in shape to each other so that a single die can be used to stamp laminations for the magnetic core from sheet metal. The use of identical laminations to form the magnetic core results in inexpensive manufacture and assembly costs for the magnetic core. Furthermore, since the laminations for the magnetic core are substantially identical to each other, the cross sectional shape of the magnetic core, once constructed from the multiple laminations, is necessarily rectangular in cross sectional shape.
One disadvantage of these previously known magnetic cores is that the cross sectional shape of the portion of the magnetic core extending through the primary and secondary coils necessarily results in a less than satisfactory magnetic coupling between the primary and secondary coils. For example, if the bobbins for the primary and secondary coil are circular in shape, a space is necessarily created between the sides and top of the magnetic core and the interior of the primary or secondary coil bobbin. Such spacing reduces the efficiency of the magnetic coupling between the primary and secondary coils.
In order to increase the efficiency of the magnetic coupling between the magnetic core and the primary and secondary coils, there have been previously known ignition systems in which the bobbins for the primary and secondary coil are essentially rectangular or square in cross sectional shape so that the interior of the inside bobbin is substantially the same size, or slightly greater than, the cross sectional size of the magnetic core. While such construction increases the efficiency of the magnetic coupling between the coils by reducing the spacing between the top and sides of the magnetic core and the coils, the use of rectangular or square bobbins for the primary and secondary coils creates manufacturing difficulties for winding the primary and secondary bobbins since it is desirable to maintain a constant tension on the wires for the primary and secondary coils during the winding operation. Such wire tension is difficult to maintain when the bobbins have a square or rectangular cross sectional shape.
A still further disadvantage of the previously known ignition coil assemblies in which the primary and secondary bobbins have a square or rectangular cross sectional shape is that the magnetic coupling between the primary and secondary coils is less efficient than obtainable if the primary and secondary coils are circular in cross sectional shape.
The present invention provides an ignition coil assembly which overcomes all of the above-mentioned disadvantages of the previously known devices.
In brief, the ignition coil assembly of the present invention is adapted to be used with a single spark plug so that each spark plug in the internal combustion engine utilizes its own ignition coil assembly. The ignition coil assembly comprises a housing which is mounted in the engine compartment.
In order to construct the ignition system assembly, the primary wire which forms the primary coil is first wound on a tubular and cylindrical primary coil bobbin on a wire winding machine. Since the primary coil bobbin is circular in cross sectional shape, the primary wire may be rapidly wound on the bobbin while maintaining a constant tension on the primary wire so that the primary coil, once wound, has a circular cross sectional shape.
With the primary coil bobbin still on the winding machine, following completion of winding the primary coil, a tubular and cylindrical secondary coil bobbin is positioned coaxially over the primary bobbin and the secondary wire which forms the secondary coil is then wound around the secondary bobbin. Following completion of winding the secondary coil, the primary and secondary coil subassembly is then positioned within a receiving cavity in the housing.
The ignition coil assembly of the present invention further comprises a magnetic core having a first portion extending coaxially through the primary and secondary coil subassembly. A second portion of the magnetic core extends exteriorly around the primary and secondary coil subassembly to the opposite ends of the first portion of the magnetic core thus completing the magnetic circuit. The entire magnetic core, furthermore, is constructed in two or more sections to enable assembly of the magnetic core to the housing.
The magnetic core is constructed from a number of steel laminations which are secured together in any conventional fashion. Unlike the previously known magnetic cores, however, the width of the laminations along the first portion of the magnetic core vary from the top to the bottom of the magnetic core such that the cross sectional shape of the first portion of the magnetic core is substantially circular and of the same size or slightly less than the inside diameter of the primary core bobbin. Thus, since the shape of the portion of the magnetic core extending through the primary and secondary coil subassembly is substantially the same as the inside diameter of the primary coil bobbin, enhanced magnetic coupling between the primary and secondary coils is obtained.
Following assembly of both the primary and secondary coil subassembly and the magnetic core to the housing, a potting material, such as epoxy, is utilized to seal the coils and other components of the ignition coil assembly to protect the ignition coil assembly from the environment.
Appropriate electrical connections are then made between the coils, an igniter, the spark plug and the engine battery.